I ended up buying a GPS "bean bag" dash mount which has a "non-skid" surface on it's underside, which works quite well on the smooth fascia of the Vantage. It managed to stay put during the Pacific Grove rally but I still have issues with reflections from the dash, which you can easily see in the two photos. Next problem to solve...

I don't think it work well at a track day (or on public roads taken at high speeds) but I can "legally" attach the camera to windscreen when I'm at the track .

The diameter of the mounting plate is big enough to use my large PanaVise mount also, which is an added benefit.
I found the mount on line at a well known store's site at less than $15 - the store symbol is a red bulls eye ...